Reason given: An explanation of why things get corrupted and how would be great. Research more things that get messed up by the glitch. Is there a way to fix the map problems effect?

Map 254 from Pokémon Yellow is a glitch location with an index number of 254 (FE).

One of map 254's appearances

When it is entered, it appears like a Glitch City. The appearance of this map may vary, for unknown reasons. The map's entrance point (which can be altered with the codes 01xxB4D3 01xxB0D3) apparently doesn't affect the appearance.

Entering map 254 may result in a freeze due to its invalid level-script pointer (D36D-D36E), which is at DC0E in WRAM.

Avoiding a freeze

Since DC0E is in WRAM, the player can easily hack in a C9 (ret) at DC0E using memory viewer to prevent map 254's freeze.

Furthermore, DC0E represents the 12th stored Pokémon in the current box's experience byte 1. While a current experience between 13172736-13238271 (C9 XX XX) is not viable, code like 00 00 C9 (an experience of 201) is. This means the player can simply place a Pokémon with 201 experience in the box as the 12th Pokémon to make map 254 non-freezing. In this example, the 00s make the code fall through to the ret.

Effects of entering map 254

Entering map 254 and avoiding the freeze has various side effects.

Easily noticeable effects

When the player enters map 254 and avoids the freeze, the cycling music will play for a moment.

The player will be warped to map hex:99, which is a house in Fuchsia City with an NPC discussing the warden and two NPCs discussing Bill. Changing the value of D364 from 254 (by tossing the quantity of item 36) to something else may be required to escape, otherwise the exit mats will take the player back to map FE.

The player's Pokédex will disappear, and the play time may become 255:59.

PC corruptions

The Prof. Oak's PC and the Hall of Fame options for a PC may disappear.

The player's stored PC items will become corrupted, and there may be more than 50 (the normal maximum amount of items) items stored.

The current Pokémon Storage System box in use may change, and the Pokémon data within boxes may change, even for boxes that aren't currently in use.

Fly-able locations corruption

The player's available fly destinations bytes (D70A, D70B) get corrupted to 42 42; which may allow the player to view the names of, or fly to glitch destinations. Making the exit mat warp to Viridian City does not change the value of D70A and D70B, and when unchanged, glitch fly destinations are possible; however, going to a place like Cerulean City (which changes the fly bytes to 4A 42) lets the player keep glitch fly destinations too.

Map problems

For unknown reasons, the scripts IDs (not the level-script pointers) of certain maps become corrupted and will freeze the game, meaning it is seemingly impossible to visit places such as the Hall of Fame. Saving and resetting does not fix this problem.

At least through cheating, it is possible to load the Hall of Fame script without the 'walking up to Oak' process by manipulating a level-script pointer of 16:6456 into D36D/E (as 5664). This can be done in Route 6, which has the 16 bank and may not freeze the game initially. Watching the credits and resetting the game will not fix the freezing problem, however, and the player would be forced to view the credits every time they load the save due to the level-script pointer not changing.

Corruption of Pallet Town events

Regardless of the whether the events near the beginning of the game were completed or not:

Professor Oak will appear outside of Pallet Town one tile west and one tile north of the sign from the player character's house. This is like a side effect of the dokokashira door glitch, except for the fact that the position he appears in that glitch may be different. Talking to Professor Oak here makes him say "Hey! Wait! Don't go out!" and an exclamation mark to appear above the player character's head, as if the player stepped in to the grass at the beginning of the game with no Pokémon, however, apparently nothing else happens.

If the player goes into Daisy's (Blue's sister's house), there will be two of her. One will be at a chair facing the Town Map, and the other will be wandering around the house. Both of her can be talked to normally, but the Town Map cannot be obtained.

If the player enters Oak's lab, there will be another copy of Oak standing near the entrance. Talking to him brings up a "?" mark on the screen for unknown reasons. The position of the ? mark depends on which position that Oak was spoken to, with higher positions having the ? mark higher up.

The rival will be in the lab and will say "Heh, my POKéMON looks a lot stronger." and one of the Pokédexes will be gone.

Talking to the northern-most Oak can have one or more effects:

If there is more than one Pokémon registered as 'own' in the Pokédex: The game makes it seems like he will rate the Pokédex as he says he will take a look at it, but the player has to talk to him again to have him rate it.

If there are less than two Pokémon registered as 'own' in the Pokédex: Professor Oak will tell the player they should talk to Pikachu to see how it feels, even if they didn't obtain the ball.

There will be a ball with a Pikachu in it, and the player can obtain it, even if the player got Pikachu before. Trying to leave the lab causes the rival to go and battle you, but the 'battle' ends before it begins. When the rival is about to leave, the game will lock-up with the rival music playing.

Other effects

Inability to battle/the game locking up before a Trainer battle until a save and reset is done.

Corruption of a fossil in Cinnabar's Pokémon Lab.

Corruption of the number of coins in the Coin Case.

Two guards appearing outside the burgled Cerulean City house, with one of them blocking the door, and the other one tile right of the previously mentioned guard.

The Rocket who stole the TM28 - Dig reappearing, and giving out the TM without a battle.

Disappearance of a Pokémon in Day Care.

Reappearance of the Rocket guarding the poster in Celadon Game Corner. They will battle (again).

How to enter it without a freeze

With a cheating device like GameShark, the player can go into a building, enter the codes 01FE64D3, 01C90EDC, then disable them. Afterwards, walking down where the exit mat is will take the player to map FE and trigger the non-freezing effect.

No cheats method

Put a Pokémon with 201 or 51456-51711 experience in the 12th position of the current Pokémon storage box; as to make the map 'non-freezing'.

Prepare Master Ball x254 (e.g. by encountering one fossil/ghost Missingno. to get 129, then by tossing 2 to get 127, then encountering it again).

Go to one of the exit mat tiles in a place like a Pokémon Center, and swap the Master Ball x254 with item 36. Make sure it is the item '(glitchblock)x(glitchblock)(glitchblock)'. If it isn't, then go to the other tile. This is important because trying to swap the Master x254 with another Master Ball seemingly doesn't work.

Go through the exit mat, and if everything worked, the player will be taken to map FE and the game won't freeze.